1. Field of the Invention
This invention relates to an improvement in an ocular refractivity measuring apparatus using a so-called photorefraction method.
2. Description of the Prior Art
A photorefraction method has been known which measures ocular refractivity from a shadow in a pupillary image of an eye to be measured. That is, on pages 545-546 in Kogaku (Optics) Volume 18, Number 10 (October, 1989), a method is disclosed in which a pupil is photographed, and ocular refractivity is estimated according to visual discrimination of a shadow in a pupillary zone, or is calculated using the gradient (the degree of inclination) of the shadow.
However, the accuracy of a conventional occular refractivity measuring apparatus using a photorefraction method is limited because it is influenced by the reflectivity of the fundus of an eye to be measured, and hence it has been difficult to obtain an exact measurement of ocular refractivity.
Furthermore, in a conventional ocular refractivity measuring apparatus using a photorefraction method, the entire apparatus must be moved to a focused position in the direction of the optical axis in order to have a clear pupillary image, and so the operation distance between an eye to be measured and the main body of the apparatus is limited. Hence, the apparatus has a poor operability because the entire apparatus must be moved.
Moreover, in a conventional apparatus, only ocular refractivity in a preset direction can be measured. If an eye to be measured has oblique astigmatism, its astigmatic angle, the degree of astigmatism and the degree of spherical ametropia cannot be measured.